It’s tempting to think of sonar as a sort of radar that works underwater. However, water is a far less compliant medium than air even for the most modern sensors, and wind conditions, temperature variations and sounds rebounding off the ocean floor can all dramatically degrade its performance. When attempting to detect the extremely quiet submarines currently in use, just a few adverse factors can turn a very difficult task into an impossible one.

This first appeared in 2016 and is being reposted due to reader interest.

Therefore, a submarine spying close to an adversary’s home port might not be able to spot another submarine heading towards it until after the collision—which can be worse than embarrassing for everyone involved.

On February 11, 1992, the USS Baton Rouge, a nuclear-powered Los Angeles–class attack submarine, was lurking twenty meters deep in the shallow waters off of Kildin Island, fourteen miles away from the Russian port of Murmansk. The Soviet Union had dissolved just two months earlier—but the Navy still wanted to closely monitor what had become of Russia’s powerful navy.

The exact nature of the Baton Rouge’s espionage activities has never been clarified. It could have involved recording the sounds produced by Russian submarines for later identification, or depositing and recovering intelligence-gathering devices.

At 8:16, something massive struck the 110-meter long Baton Rouge from below, scratching the nuclear-powered submarine’s hull and causing tears in its port ballast tank. Fortunately, the American submarine’s hull was not further compromised.

It turned out a Russian Sierra-class nuclear-powered attack submarine, the B-276 Kostroma, had attempted to surface underneath the American submarine. Swimming at around eight miles per hour, the Russian boat’s conning tower had impacted the belly of the American ship. The titanium-hulled Kostroma’s sail was partially crushed from impacting the Baton Rouge’s belly, and pieces of the American submarine’s anti-sonar tiles were later found embedded in its surface.

Both submarines were designed to launch cruise missiles from their torpedo tubes, some of which could theoretically be armed with nuclear warheads. However, Russia and the United States had recently agreed to withdraw such warheads under the START I treaty, and it was likely that the Baton Rouge at least no longer carried them. Still, a worse collision could have breached the reactors on either vessel, irradiating the submarines and the surrounding waters.

Fortunately, this did not occur. The Baton Rouge circled around and contacted the other submarine to make sure it wasn’t in need of assistance, and then both vessels returned to port for repairs.

The accident caused one of the United States’ first diplomatic incidents with the newborn Russian government, with Secretary of State James Baker having to meet in person with Yeltsin and assure him that the United States would scale back its spying in Russian waters, a message belied the following year by another submarine collision off the Kola peninsula.

The incident also highlighted differences on the definition of “international waters.” The United States follows the standard of measuring them twelve miles away from the nearest landmass. The Baton Rouge was in compliance with this principle. Moscow, however, defined them as extending twelve miles from a line formed by the two sides of a gulf, by which standard it considered the Baton Rouge in violation of its territorial waters.

The second in the prolific Los Angeles class, the BatonRouge was only seventeen years old. However, the cost of repairing the 110-meter-long vessel, combined with the already scheduled expenses of nuclear refueling, was judged excessive and the boat was decommissioned in January 1995.

The Kostroma, however, was repaired and put back to sea by 1997, and remains active to this day. Russian sailors have painted a kill marking on its conning tower to commemorate the “defeat” of the Baton Rouge.

Stealth in Shallow Water

How did this accident even happen? Some articles in the press characterized the subs as having been involved in a cat-and-mouse game that had gone too far. Indeed, such games were common between the attack submarines of rival nations, and had resulted in collisions in the past.

However, that account remains unlikely because a submarine can only play a cat-and-mouse game if it is able to detect the other ship. And in the shallow waters off of Kildin Island, it is unlikely either vessel could.

This is because in shallow water, breaking waves create at least ten times the background interference for sonar operators, making it extremely hard to discern a submarine’s quiet propeller screw. Furthermore, even signals that are detected will have reflected off the ocean floor and the surf so that it would become difficult to isolate them against the background interference.

Analyst Eugene Miasnikov calculated in 1993 that the detection range using passive sonar of a slow-moving Sierra-class submarine in such a noisy environment would likely have been between one hundred and two hundred meters, or fewer if it was a windy day. And detection range might have fallen to zero if the Russian sub approached from a sixty-degree arc behind the Baton Rouge, which is not covered by the submarine’s fixed sonar array.

The Russian submarine would also have had little chance of detecting the quieter Los Angeles–class submarine. More powerful fixed antisubmarine sensors might only have been effective at ranges of three to five kilometers in such conditions, too short to reach the Baton Rouge’s position. Submarines can also deploy towed sonar arrays behind them to increase their sonar coverage, but these are difficult to control in shallow waters and were therefore not in use during the incident.

A submarine or surface ship could also use active sonar to emit sound waves that would reflect off another submarine’s hull. In shallow water, this might have increased detection ranges to a few kilometers. However, doing so would also reveal the platform using the active sonar.

The Baton Rouge surely did not use active sonar so as to remain undetected. Nor did it detect active sonar from the Kostroma. Thus, neither vessel was using active sonar, and their passive sonars were likely not strong enough to detect the other in the noisy shallows.

This explains why submarines measuring longer than a football field in length can run into each other, oblivious to the other’s presence until the crunch of impact. As evidenced by the alarming collision in 2009 between the nuclear missile–armed French Triomphant and the British Vanguard, the risks of underwater collisions between nuclear submarines remain quite real today.

Sébastien Roblin holds a Master’s Degree in Conflict Resolution from Georgetown University and served as a university instructor for the Peace Corps in China. He has also worked in education, editing, and refugee resettlement in France and the United States. He currently writes on security and military history forWar Is Boring.